This is one of those things that you either love or you hate. There is no in between. I have known a few people who thought they were great, but most people don't like them.

Pro's.
Easy to repair. Geez, it's cement already.
Will take a beating.

That's about it for the pros.

Cons,
The smaller the boat, the heavier it is for it's length. There is no weight advantage until you get above 40 feet or so.

They rust. The interior of the hull of a ferro cement boat is rebar and chicken wire. That is what holds it together and provides strength. But cement absorbs water. Eventually the rebar and wire rust and it seeps out through the cement.

Most of the ones I have seen are not very professionally designed (the hull) and look rather clunky but that's just my opinion.

Insuring them can be a problem.

It's really difficult to get a nice yacht quality finish. They always seem to look a little rough.

From what I've heard and I think it makes sense is that: ferro cement is not a backyard friendly medium for boat building which is where most were built. If the steel starts to rust which it will if the hull is not sealed perfectly the integrity of the hull is gone. Stopping rust once it's started is very difficult.

That said I have heard of professionally ferro boats performing very well for decades. So I'd say you've got to be very wary if looking to purchase one.

A few things that I ran into -- it's easy for previous bad patches to be hidden on ferro boats. As I dug into mine I found three spots that had sustained piecing damage probably from being hit by another boat and discovered that although it looked fine on the surface that no PVA (poly vinyl acetate) glue had been used to bond the two cements AND no new wire mesh (the ferro part) had been woven into the existing structure to maintain strength.

Also, while trying to resurface the deck (also cement) I found that it had in some places up to an inch of various coatings that had been added over the years in an attempt to cover an uneven original surface. It was a mess and in places I had found that the whole deck (the cement itself) was totally rotten. The things that had been added to the surface had created a sort of reverse water seal that kept the cement wet 24/7. This allowed bio material and later bugs to take up residence in the cement and finally into the plywood onto which the original deck had been laid.

The point I'm trying to make is that although they are a time-proven material for building boats, they also hide problems really well. On the surface everything looked solid and dry. Oh yeah, and I eventually lost the boat in a hurricane which since I was 3 months into looking for an insurer who would touch a ferro boat, was uninsured.

Luckily I only had a few thousand into it, but had I known more about ferro boats, I would definitely still have that 3 grand in the boat fund.

Here's my ferrocement boat. The Pleasure Vessel
The only problem I've had with it is the fact that I know nothing about sailing yet and have no crew.
By the way, these boats are relatively cheap and third party insurance, and only when certain places demand it, can be got, otherwise why bother spending thousands each year to insure a boat that didn't cost an arm and a leg to begin with. Just an arm.

My last boat was a J. Benford designed 23' canoe yawl made of ferrocement. It was owner-built but the plastering was contracted out to professionals. It was virtually impossible to tell it was cement just by looking. Yes it was heavy but the displacement was the same as it would have been for that design regardless of material. It's just that in the ferro version less of this weight was in the keel. The hull was only 1/2" thick and the armature was NOT rebar and chicken wire. It was made of SS rods mounted vertically and horizontally on 6" centers. This was then covered with SS hardware cloth with 1/4" openings.

The only problem we ever had was getting paint to stick to the hull and so, had to repaint every couple of years. But that just gave us a chance to entertain the other marina dwellers who wondered what color we'd come back from the yard with THIS time.

Best thing about a ferro boat, if it's good one: they're cheap. Worst thing: you're not going to get much back when/if you sell her.

Steve
I don't agree with the 'you're not going to get much back if you sell her'.
Mine was bought for a fair price and there's no way I'd sell her for less than I bought her for because the price was fair to start with. I was just looking at a bunch of ferro boats for sale and was pleasantly surprised to see the average price of a 45 footer to be quite a bit more than I originally paid for mine. I could even make money if I sold my boat!

See! They're not that bad, are they! I notice that one of the main grudges against a ferro is 'not being able to get insurance', 'low resale value', but if you compare the initial investment to 'normal' boats, with the latter usually demanding some kind of insurance because the initial investment was not insignificant, then I think you'll find that a ferro boat isn't such a white elephant after awl.